SSVs are safety devices mounted deep within wells to control flow to the surface. They generally have many components in common. The valve member is generally a flapper, which rotates 90 degrees and is held open by a flow tube, which is shiftable downwardly to turn the flapper 90 degree to move it away from a closure or seat. A control system is generally employed involving hydraulic pressure from the surface connected to the SSV below. In general, applied pressure opens the valve, while removal of applied pressure from the surface allows a spring acting on the flow tube to move the flow tube upwardly so that the flapper can pivot 90 degrees to a closed position.
Various types of control systems have been employed. To reduce the size of the closure spring acting on the flow tube, chambers pressurized with a gas have been used to counteract the hydrostatic pressure from the column of hydraulic fluid in the control line that runs from the surface down to the SSV. Since the pressurized gas resists the hydrostatic force and offsets it, closure of the SSV is accomplished with a fairly small spring when the actuating piston, acting on the flow tube, is placed in hydraulic pressure balance, thus allowing the small closure spring to shift the flow tube and allow the flapper of the SSV to close.
With the advent of use of pressurized chambers having a gas on top of hydraulic liquid acting on the opposite side of an operating piston from the control line hydrostatic pressure, numerous seals had to be used. A concern then arose as to the operation of the control system if one or another of the seals in the system failed to operate properly and permitted a leakage in one direction or another. Fairly complex designs were developed to try to compensate for failure of system seals in a manner that would allow the SSV to fail in the closed position. Some of these complex systems to obtain failsafe closure in one or two failure modes, but not necessarily all or even most failure modes, are illustrated in U.S. Pat. Nos. 4,660,646 and 5,310,004. Other control systems for SSVs employing pressurized chambers would, incidentally, go to a fail-closed position in the event certain seals in the system leaked. However, such designs were not put together with the idea of ensuring that the valve would go to its failsafe closed position in the event of malfunction of most or all of a number of given system components. Typical designs showing pressurized chambers, in conjunction with control systems for SSVs, are illustrated in U.S. Pat. Nos. 5,564,501 and 4,676,307. Also of general interest in the area of SSV control systems are U.S. Pat. Nos. 4,252,197 and 4,448,254. A specific control system described in this application and useful with the annulus pressure sensing feature is shown in U.S. Pat. No. 6,109,351. FIGS. 1-3 of this application are the prior art FIGS. 1-3 of U.S. Pat. No. 6,109,351 for use as background of a control system where the annulus pressure-sensing feature of the present invention would be particularly useful. Other control systems for SSVs are also amenable to use of the present invention. One example of which is U.S. Pat. No. 6,173,785 illustrating a pressure-balanced system. FIG. 5 in this application shows how the present invention is applied to a balanced control system shown in that patent.
What has been lacking in these control systems is a simple design which will serve to allow normal opening and closing of the SSV while, at the same time, allow the valve to fail in the pre-designated safe position in the event of an occurrence of numerous different events relating to component failures in the control system. One of these failure modes is a rupture of the control line and the tubing, which would suddenly allow tubing pressure into the annulus and the control line. It is, thus, the object of the present invention to present a simplified control system for normal functioning of an SSV between an open and closed position. It is another object of the present invention to configure the control system so that if many of its components should happen to fail, the system will either immediately or eventually, in the event of slow leaks, go to its failsafe position. It is another object of the present invention to designate the closed position of the valve as the failsafe position so that failure of many different seals within the system, which can result in leakage into or out of the control system, will result in failure, which allows the SSV to go to its desired fail-closed position. Additionally, the control system has the objective of allowing the SSV to close if the control line is suddenly exposed to elevated annulus pressure resulting from a rupture of the tubing and the control line. These and other objectives will become more apparent to those skilled in the art from a review of the preferred embodiment described below.